1. Field
Example embodiments of the present invention generally relate to a fixing device and/or a method and/or device for heating control used in the same; for example to a fixing device and/or a method and/or device for controlling, by pulse width modulation, a heating mechanism capable of stably driving the heating mechanism and reducing errors due to unstable operation of the heating mechanism.
2. Discussion of the Background
Typically, an image forming apparatus includes a heating system with a heat source used for a particular purpose. The heating system is provided at a fixing unit, a drying unit, or a data erasing unit of the image forming apparatus and used to supply heat for fixing or drying a formed image, or erasing image data.
For example, the heating system is used to dry an image formed with ink in the drying unit of an inkjet printer, and to erase image data on a rewritable medium in the erasing unit of some types of image forming apparatuses.
The heating system is also used to fix a toner image onto a recording medium in the fixing unit of known electrophotographic apparatuses, including printers, copiers, and facsimiles.
In a related art electrophotographic apparatus, the fixing unit includes a heater, a fixing member, and a rotating member, and fixes a layer of toner on a recording medium by melting toner particles with heat and pressure.
Specifically, a first step of fixing the layer of toner on the recording medium is to provide electricity to the heater generating heat. The heater provides heat to the fixing member and maintains the temperature of the fixing member at a desired value. Then, the recording medium is passed through a nip formed between the fixing member and the rotating member, in which the layer of toner is melted with heat and pressure.
In this fixing process, the temperature of the fixing member needs to be adjusted to the desired value because a deviation from the desired value may lead to degradation of image quality.
The degradation of image quality is caused by, for example, adhesion of toner to the fixing member. When the temperature of the fixing member becomes too high, excessive heat is applied to the layer of toner, causing melted toner particles to adhere to the fixing member.
Furthermore, insufficient melting or unstable fixing of toner on the recording medium may also degrade image quality. When the temperature of the fixing member becomes too low, the layer of toner is not provided with sufficient heat and fails to stabilize on the recording medium.
Therefore, the temperature of the fixing member needs to be controlled to avoid such degradation of image quality. In addition, with recent advances in production of color and/or high definition image with high glossiness, which is easily affected by temperature, accuracy in the temperature control of the fixing member becomes more important.
Another issue concerning the temperature control of the fixing member relates to a reduction of waiting time. The waiting time is a length of time that a user has to wait until the electrophotographic apparatus becomes available after switch-on.
One of important factors to reduce the waiting time is a reduction of warm-up time, i.e., an amount of time required to warm up the fixing member to the desired temperature from a cold state. The warm-up time or the waiting time can be reduced by swiftly adjusting the temperature of the fixing member to the desired value. Therefore, the temperature control of the fixing member needs to be performed with swiftness as well as accuracy.
A common technique to reduce the warm-up time is to reduce heat capacity of the fixing member by forming a part of the fixing member, such as a film and a rotating body, with a material of low heat capacity.
For example, a rotation type fixing unit includes a film of low heat capacity. The film is provided to form a nip with a rotating body in which heat is applied to the recording medium.
Another example is a belt-shaped fixing unit including a rotating body of low heat capacity. The belt-shaped fixing unit has two or more rotating bodies, including first and second rotating bodies, and a belt stretched across the rotating bodies.
In the belt-shaped fixing unit, the first rotating body has low heat capacity, and is opposed to an additional rotating body to form a nip. The second rotating body contains a heater inside. The belt transfers heat generated by the heater from the second rotating body towards the first rotating body so as to melt and fix the layer of toner at the nip.
The temperature of the fixing member of low heat capacity rapidly reacts to a change in heat supplied from the heater, reducing time constant in the temperature control. To accurately control the temperature of the fixing member with the reduced time constant, the heat needs to be supplied to the fixing member without a deviation in time or a time lag.
Commonly, the heat supply to the fixing unit from the heater is controlled by increasing or decreasing an amount of supply voltage to the heater. Various methods have been introduced to control temperature through the supply voltage.
One of these methods is an on-off control. In the on-off control, a temperature sensor (e.g., a thermopile or a thermistor) detects the temperature of an object and compares the detected temperature with a set temperature. The heater is switched on and off according to a difference between the temperatures. Namely, electricity is supplied when the detected temperature is below the set temperature, and not supplied when the detected temperature exceeds the set temperature. An amount of heat corresponding to the supply voltage is transmitted from the heater.
The on-off control has a drawback since accurate temperature control is difficult due to dead-time, i.e., time delays in transferring heat between components or detecting temperature of the object, for example. The dead-time induces fluctuations in controlled temperature, referred to as a temperature ripple.
Other examples of the control methods are a proportional-integral-derivative (PID) control and a proportional-integral (PI) control. In the PID or PI control, a temperature sensor detects the temperature of an object and transmits the detected temperature to a compensator. The compensator uses the detected temperature to calculate and output a manipulated variable. The manipulated variable is controlled to an optimum value so that the detected temperature is brought to a setpoint.
For example, in the PID control, the manipulated variable is the sum of proportional, integral, and derivative control actions. The proportional action is proportional to a difference between the detected temperature and the setpoint. The integral action is proportional to an integral of the difference between the detected temperature and the setpoint. The derivative action is proportional to a derivative of the difference between the detected temperature and the setpoint.
The PID control is advantageous to the on-off control, but may also cause deviations in controlled temperature due to the dead-time when applied to a system with a low time constant, referred to as an overshoot and an undershoot.
For obtaining accuracy in the temperature control with a low time constant, these fluctuations and/or deviations of controlled temperature needs to be reduced by compensating for the dead-time.
Background techniques have been developed to perform compensation of the dead-time in the temperature control.
For example, in a background temperature control method, a controller calculates an estimated temperature for a next control timing using an average variation rate of measured temperature. The average variation rate of measured temperature is obtained by dividing a difference between a first temperature and a second temperature measured immediately before the first temperature by a time interval. When the estimated temperature is below or above a target value, the controller changes electricity supplied to a heater by increasing or decreasing an on/off ratio (i.e., a duty cycle) of the electricity supply.
This background temperature control method may fail to eliminate the temperature ripple due to the dead-time that occurs between detection of temperature and driving of the heater when the estimated temperature is substantially deviated from the target value. Namely, this background temperature control method does not meet the dead-time that is generated throughout the fixing process.
In another background temperature control method, the dead-time is compensated with a model in a limited manner. Namely, the dead-time compensation is performed during a temperature adjustment in response to a deviation from a desired value, and is not performed in other cases. A drawback of this temperature control method is that the dead-time compensation with a model may result in errors due to modeling deficiencies under unexpected conditions, such as a change in dead-time caused by disturbances and a significant error in driving the heater.